join passwords

TODO:

• Used challenge slots can fill up by people trying to join and failing, and then they'll start being reclaimed in a random order. Change this to ensure a free slot is pulled if available, and if not grab the oldest slot.
• There's no message for an incorrect password right now. It just shows the password entry screen again. Show an incorrect password message.
• Add a command to disable join passwords after enabling them
• uhhh move some of the functions to not-dumb places in the files I guess?
• Password is currently the last thing checked before joining. If I can easily put it before downloads I will